Shoe soles, e.g., athletic shoe soles, must be flexible and typically include a patterned, ground engaging, tread layer or outsole, a midsole consisting of highly resilient porous material, and an innersole. The midsole of athletic shoes is typically formed of a softly elastic material such as foamed polyurethane or an ethylene-vinyl acetate copolymer. A softly elastic material is provided to absorb impact associated with movement of the user's foot such as walking, running, jumping, etc., and to provide a cushioning effect to the user's foot.
Although shoe midsoles must be flexible, they must nonetheless be stable to prevent a less than ideal foot strike which often results in over-supination or over-pronation of the user's stride. Over-supination or over-pronation occurs when the user's foot strikes the ground at an angle relative to the horizontal axis. Stated differently, the inside or outside of the user's foot bears the impact as the foot strikes the ground as opposed to the impact being uniformly distributed across the user's foot. This, in turn, may result in injuries to the user's joints.
It is desirable for shoe soles to relieve stresses that are imposed on a human foot during activities such as athletic activities by providing a spring arrangement. The spring arrangement stores and releases energy caused by compression of the midsole when portions of the foot (i.e., the heel and ball of the foot) impact upon the ground. The stored energy, which compresses the midsole, may be returned to the user's benefit while also assisting to avoid overuse injuries caused by repetitive impact to the body joints as the foot strikes the ground. A problem associated with shoe midsoles is wear due to creep. Creep is caused by strain experienced by the sole, e.g. the midsole, when subjected to repetitive, applied stress and results in a permanent deformation and loss of resiliency in the midsole. Creep may be particularly imminent with a less than ideal foot strike, e.g., supination or pronation of the foot. As a result, the midsole may no longer be flat when only a portion of the midsole experiences creep due to uneven repetitive stress. In turn, the useful life of the midsole is diminished thereby decreasing the life of the shoe.